Reaction of 1-aryl-3-chloropropenes with Grignard reagents. Nucleophilic substitution versus single-electron transfer

Abstract

The reactions of ambident 1-aryl-3-chloropropenes (1a–;e) with a series of Grignard reagents, R′MgY (R′= Me, Pr, Ph, Pri, But; Y = Br, I), were carried out in diethyl ether (EE) and tetrahydrofuran (THF). The products were a mixture of two alkylation products (2) and (3), and three dimerization products (4)–;(6). The alkylation: dimerization ratio and the composition of the two alkylation products were a marked function of substitutent electronic effects in the chlorides (1a–;e), R′ or Y of R′MgY, and solvent. On the basis of the stereochemistry of alkylation, cyclizable probe experiments, and the effect of the addition of FeCl3 on product composition, the following conclusions were obtained. First, dimers (4)–;(6) are most likely to be produced by a mechanism involving single-electron transfer (SET). Second, for the formation of alkylation products (2) and (3), three alternative pathways contribute depending on the nature of R′MgY and solvent, (a) competitive SN2–SN2′ pathways in the reaction of R′MgBr in EE, (b) a process involving SET in the reaction with R′Mgl in EE, and (c)SN2 pathways in the reaction of R′MgBr in THF.